1. Field of the Invention
The present invention relates to a zoom lens and a camera using the zoom lens and, more particularly, to a zoom lens of high optical performance most suitable for a camera having a comparatively short back focus among lens shutter cameras or the like, and to a camera using the zoom lens.
2. Related Background Art
In recent years, with the miniaturization of optical devices such as lens shutter cameras, video cameras and digital cameras, there has been an increasing demand for smaller zoom lenses of a high variable magnifying power and of a shorter entire length as a photographing lens for use in such optical devices.
As a means for miniaturizing the photographing lenses, so-called positive lead type zoom lenses having a lens unit of a positive refractive power placed at the foremost position are ordinarily used. This type of zoom lens has the capability of reducing the back focus as its significant feature and is used particularly effectively in a lens shutter camera or the like which does not require a space such as that in a single lens reflex camera for placement of a quick return mirror at the rear of a lens system.
As a positive-lead zoom lens having a short back focus, a positive/negative-two-unit zoom lens is known which is formed of a first lens unit having a positive refractive power and a second lens unit having a negative refractive power, and in which the two lens units are moved to change the magnifying power. This zoom lens is simple in mechanical construction but has drawbacks in that if the variable magnifying power is increased, variation in chromatic aberration becomes large and it is difficult to correct an image plane displacement caused by magnifying power change, because only the two lens units exist and because the degree of lens design freedom is low. In the positive/negative-two-lens-unit zoom lens, therefore, the variable magnifying power ratio is limited to about 3 to 3.5 in the case of a design for achieving both a reduced size and improved optical performance.
Therefore, to obtain a zoom lens having a variable magnifying power ratio of about 4 to 5, it is necessary to adopt a lens unit construction using three units or more, i.e., the basic positive/negative-two-unit zoom lens with at least one additional lens unit.
Three-unit zoom lens formed of three lens units respectively having a positive refractive power, a positive refractive power and a negative refractive power has been proposed as a zoom lens having a comparatively short back focus and suitable for magnifying power change at a high ratio as compared with the conventional one.
Various four-unit zoom lenses formed by adding a lens unit of a comparatively low refractive power to a three-unit zoom lens have also been proposed.
The assignee of the present application proposed three-unit zoom lenses and four-unit zoom lenses in JP 06-214157 A, JP 06-214158 A, JP 06-222267 A, JP 06-250087 A, and JP 06-250088 A (five publications above correspond to U.S. Pat. No. 5,815,320), and in JP 06-294932 A (corresponding to U.S. Pat. No. 5,831,772). Each of the zoom lenses disclosed in these publications is a four-unit zoom lens having in order, from the object side, a positive refractive power, a positive or negative refractive power, a positive refractive power, and a negative refractive power, or a three-unit zoom lens having a positive refractive power, a positive refractive power, and a negative refractive power.
Other four-unit zoom lenses having in order, from the object side, a positive refractive power, a negative refractive power, a positive refractive power, and a negative refractive power have also been proposed in JP 2,579,215 (corresponding to U.S. Pat. No. 5,004,329), JP 10-301027 A, and U.S. Pat. No. 4,822,152, U.S. Pat. No. 5,272,566, U.S. Pat. No. 5,172,273, and U.S. Pat. No. 5,170,292.
For example, the method of increasing the refractive power of lens units and increasing the extent of movement of each magnifying power-changing lens unit is generally known as a method of increasing the variable zoom ratio of a zoom lens while reducing the size of the zoom lens.
However, if the method of increasing the refractive power of lens units and also increasing the extent of movement of each magnifying power-changing lens unit is simply used, variations in aberrations accompanying a change in magnifying power are increased and it is difficult to obtain improved optical performance over the entire variable magnifying power range.
In view of the above-described conventional art, an object of the present invention is to provide a zoom lens of a novel construction capable of changing the magnifying power as desired while maintaining good optical performance.
To achieve the above-described object, according to one aspect of the present invention, there is provided a zoom lens having a first lens unit having a positive optical power, a second lens unit, a third lens unit, and a fourth lens unit having a negative optical power, the first to fourth lens units being placed in this order from an object side to an image side. In this zoom lens, zooming is performed by moving the lens units so that the spacing between the first lens unit and the second lens unit at the telephoto end is larger than the spacing between the first lens unit and the second lens unit at the wide-angle end, and so that the spacing between the third lens unit and the fourth lens unit at the telephoto end is smaller than the spacing between the third lens unit and the fourth lens unit at the wide-angle end. The power and the placement of the respective lens units constituting the zoom lens described above, the lens construction of each lens unit, lens shapes, lens materials, etc., are determined from various viewpoints, as represented by way of preferred examples thereof explained in the following description of embodiments of the invention.